HT has been working on frame-rate optimization for the next beta. As a result of the changes that he's been making, Beta 19 will require DirectX 9. If you are using an older version of DirectX, you will need to upgrade your DirectX version to run the next beta release. If you are using an older set of video drivers, you may be required to update your video drivers as well.
Either Beta 19 or Beta 20 will be a substantially large update as we plan to enable all or nearly all of the remaining planes and vehicles.